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1. Relative humidity measures the amount of moisture in the air. It is expressed in a percentage of how much moisture the air could possibly hold. The wetter or damper the air is, the higher the relative humidity. The drier the air feels, the lower the relative humidity. Thus, 100% humidity is actually rain.

3. When closing the container, the humidity is contained in a very small space. As the container is moved from one climate to another, the humidity vaporizes up into the ceiling of the container, where it will start to rain down onto the cargo. Either the humidity will start to soak into the cargo or run along the surface of i e steel products. It can also turn into water that runs towards the bottom of the container. InterDry Power Desiccant actually “catches” the moisture before it vaporizes.

4. As soon as the climate changes, the moisture will either vaporize again or really start making trouble among your different products being shipped, if you do not use a desiccant. The longer the transportation time – the more damage may possibly develop.

5. You may have seen your cargo being loaded in perfect condition. But have you ever opened a container full of molded furniture? Or rusty motor equipment? Or rice sacks full of mildew? Or fruit covered in fungus? The customers do not like it. They will blame you. They will lose money and time, they will lose their customers and they will make claims to you. Then you will lose money and time, and possibly even the client. Since there are many parties along the transportation chain, many may be blamed for damaged goods – like manufacturers, traders, shipping lines and forwarders. Why be among them?

6. Other desiccants being used are i.e. silica gel. Silica gel is the most common type of desiccant in use today. It is a porous sand and can absorb moisture in the air, often up to 40 % of it´s weight. However, silica gel absorbs moisture best in small, confined spaces and often end up getting saturated in a very short time span, making it unsuitable for container shipments. When saturated, it will not help catching the moisture anymore. Be aware that some silica gel – the blue contains cobolt – is toxic, and can not be disposed of any which way.

A container is a closed system with it’s own ”weather” inside. It differs from the warehouse in that the variation in temperature is much greater. It is not unusual to see containers wherein temperatures range from freezing to 60-70C during the course of a single voyage.

The central fact is that warm air can hold more moisture than cold air. That means that if warm air is cooled, it becomes more humid. And if it is cooled enough, some of the moisture must rain out – condense. That is exactly the same phenomenon that causes dew in the grass or fog on a cool autumn.

In a container a fast temperature change of 5-10C is often enough to cause problems. Water will condense on the coolest available surface, which is often the container ceiling or walls. From there it may drip down onto the cargo and cause damage – “container rain”. At other times it condenses on the cargo, – “cargo sweat”-, which is usually even more damaging

Even without any condensation, elevated humidity over a period of time is sufficient to cause damage. Many metals will corrode or discolor at a rather modest level of humidity , 60-70%. At higher levels of humidity, 80%-90%, moulds will grow, labels will peel and corrugated boxes will start to soften.

The Relative Humidity (RH) is a percentage measure of how much moisture the air holds as compared to the maximum mount of moisture air at that temperature can hold. That means that completely dry air has a RH of 0%. The RH can never be more than 100%, or any excess moisture will rain out. There is little danger of damage to anything if the RH is below 50% or so.

The Humidity Changes when the Temperature does

The important thing to realize is that the humidity of the air changes only as a result of the change in temperature. When air cools it becomes more humid, – even though the moisture content in the air remains the same.

The Humidity in a container will go up and down throughout the voyage, as a result of changing temperature only. If the temperature changes rapidly enough there is sure to be moisture trouble, even if the container may be fairly dry by reasonable standards.

In a container, moisture evaporates into the air during periods when the container is warm. The warm dry air can accept a lot of moisture. Or warm moisture containing air enters from the outside through “Container Breathing”. When the container cools down, that air becomes very humid. And it is then the troubles start.

But the temperature doesn’t have to vary in time to create a difference. It is equally bad when different parts of a container are at different temperatures. When warm air moves into a colder part it becomes humid and perhaps even condenses moisture. Tons of moisture can be redistributed within a container during a voyage through such processes. Mysterious patterns of damage may arise, such as mold only in certain parts of the cargo.

Temperature changes in a container may arise because one side of the container is exposed to the elements and another is not. Or it may arise simply as a result of a great thermal inertia in the cargo as outside temperatures change. It is common that it takes weeks for the temperature to equalize through a densely stuffed cargo.

It should be noted that all the basic processes outlined above are strongly nonlinear. A small difference in conditions may cause a grate difference in outcome. That is why the pattern of damage may seem unpredictable.

Where Does the Moisture in the Container Come From?
The moisture in the container:
* Is in the air when the container doors are closed
* Is contained in the cargo and packaging and is evaporated throughout the voyage
* Enters from the outside through so called container breathing.

The amount of air contained in the air at loading depends at the temperature and the humidity at loading. If loading at cool temperatures the amount is seldom significant, at most a few hundred grams. At loading in the tropics, however, the total amount of moisture could be a Kg or more.

Most cargo and packaging materials can both absorb and evaporate moisture. What happens depends on the temperature and how humid the surrounding air is. It is common that the cargo will evaporate during one part of the voyage and absorb during a different part.

No container is airtight. Moisture can move both into and out of the container as a result of temperature variations. Unfortunately, common circumstances will lead to a gradual build up of moisture within the container.

It could very well happen that you start with a very dry cargo, but at some later time the cargo has absorbed a lot of moisture which may be released in a very destructive way. If there is a temperature difference within the cargo, very substantial amounts of moisture may be re-distributed within the cargo. The moisture will always move from the warmer to the colder part.

Any absorbers put in the container are of course expected to be part of the solution and not the problem. Alas, that is not so. Unfortunately almost all kinds of absorbers, other than Absorpole and Absorbag, will re-evaporate moisture under some circumstances, usually in connection with a period of elevated temperature some time into the voyage.

Container Breathing
No container is airtight. If the seals are good and the vents are taped shut, air will move in and out more slowly, but any pressure differential between inside and outside will certainly be equalized in a matter of hours.

The air pressure outside a container will vary for metrological reasons over the course of a voyage. When the barometer falls, air and moisture will move out from the container, and when it rises the reverse will happen.

This effect becomes much more significant if the container is subject to repeated cycles of large temperature variations. When the container cools, the pressure inside it goes down. Air and moisture from the outside will move in until the pressure is equalized. When the container heats up, the reverse happens.

While moisture can move both in and out of the container, it is not a balanced process. Under very common circumstances, cycles of temperature variations will lead to a build-up of moisture within the container.

If the container contains absorbent packaging material, that build-up can be very significant indeed.

Moisture Exchange of Packages within the Container
A package is like a container in miniature. Even where it completely sealed, there could still be moisture damage inside as a result of temperature changes alone. In fact, most packages exchange a lot of moisture with the air inside the container. Almost all common plastics, except alu-foil, let moisture diffuse through to a significant degree as will coated or uncoated cardboard. The least mistake in sealing a plastic package will anyway leave it subject to “breathing” processes.

For a plastic wrapped package, including a pallet liberally shrink wrapped, the most important process of exchange is diffusion through the plastic. The diffusion rate is proportional to the surface area of a package. Thus it is important to note that a bigger package has a smaller surface area in relation to its volume, than does a small package. When you put many boxes into a pallet, or stuff many pallets closely together, you lessen the significance of moisture diffusion.

For a wooden crate, diffusion as such may be of less importance in the timeframe of a typical voyage, but the natural breathing of the wood may be a dominant mechanism. If not, the “breathing” will be the most important aspect. The breathing is proportional to the amount of free air inside the crate and it is exponentially dependent on the temperature outside at constant relative humidity.

It is worth noting that moisture will not only move into the packages, but also out of them if the container environment is sufficiently dry. In practice it is often found that it makes a more sense to install moisture protection in the container and leave the pallets open at top and bottom to breathe, than to attempt to seal out the moisture.

Sea Containers are an economical and safe way of shipping almost any kind of cargo. But putting a cargo into a closed strong box also entails a constant risk of moisture damage for every kind of cargo on every voyage.

Metals corrode, discolour and loose their shine

Cargo and packaging get moldy, soft, crumbled and discoloured.

Bad smell

Physical damage from water, ice, things gluing together, caking etc.

Such damage may result in substantial losses and costs. Yet obviously not every shipment suffers moisture damage, and most of those that do, suffer only limited damage. In fact, lots of moisture damage remains unrecognized, because it is considered “normal”. Very few shippers have a good system of feedback from the receivers of their goods. There may be lots of things they don’t know.

The pattern of moisture damage may seem random. The moisture processes are examples of strongly non-linear physics. That means that very small differences in the cargo and voyage conditions can have a huge effect of the outcome. That is why you may have 4 perfectly safe shipments and the 5th may be a disaster. This means that there is always a risk of moisture damage in the next shipment, even if the last one was ok.

Moisture Damage can be Prevented

All containers contain moisture from the time of loading and in the cargo. No container is airtight. Moisture will move in and out of the container during the course of the voyage – “Container Breathing”.

The objective of a moisture protection program is to prevent the build up of moisture in the air to levels where it may cause damage. This is done by reducing the amount of the moisture entering the container and by using “Absorbers” to remove moisture from the air.

We like to present the InterDry Power Desiccant moisture protection program as a kind of checklist of things that should be arranged as well as possible. And it is to be noted that many of the items on the list can be influenced only to a degree. Yet even small changes can result in big improvements. In some cases a few tens of grams of water in the wrong place is enough to cause significant damage.

Storing pallets inside or outside is often enough to make the difference between no damage and “disaster”. Simply adjusting the temperature of the cargo at loading can prevent damage. Thus it is well worth to make what improvements are at all practical, and the balance will then have to be taken up by the packaging and the absorbers.

Is the Container Tight?

A minimum requirement is of course that the container is watertight against rain and spray. That is usually the case, but especially the bottom side and the doors are vulnerable to damage that may not be noticed.

Check the seals. Certainly no container is airtight, but a container in good condition allows air (and moisture) to move in and out of the container only slowly, over hours perhaps. That significantly reduces the amount of moisture moving into the container under common circumstances. (Container Breathing)

Tape the vent holes if you are shipping a dry cargo. For a moist cargo, such as agricultural commodities, it is usually better to leave the vent holes open.

Is the Container Dry?

A container that has been washed before loading, brought in from outside into a warm loading area or stored in a humid place, may contain lots of water. In particular, attention must be paid to the container floor. The humidity of the wood should not be above 18%.

All pallets and other wooden dunnage must be dry. Preferably the moisture content should not above 18% and certainly not above 20%. It is easy to check the humidity with a small handheld device commonly used in the construction industry and costing a couple of hundred euros.

CONDENSATION DAMAGE

Is the Cargo Dry?

Some cargoes are entirely dry and don’t contain any moisture, e.g. pure metal products without corrosion protection or surface treatment.

But most cargoes contain moisture, if only in the packaging that usually include wood, cartons, paper etc. Most of this moisture is bound in the material and is not easily released, but even a small percentage can cause problems. Even if the product appears dry at loading is no guarantee against things going wrong in a container where temperatures may later on reach 60-70C.

Some cargoes unavoidably contain a large mounts of moisture, even after having been made as dry as practical. They require a more sophisticated moisture protection installation.

Is the Loading Dry?

You can easily destroy all the other precautions you have taken by loading the container under the wrong conditions.

Particular attention should be given to the storage of containers, pallets and dunnage. Not to speak of the cargo. Even under a tarpaulin dry pallets or crates stored outside, can quickly absorb significant moisture.

Wet or snowy tarpaulins, truck wheels or even shoes may introduce a lot of water into the container. Again beware of wood, including the container floor, that may look dry but in reality be very moist.

A more subtle consideration is to make the cargo have the same temperature as the container during and after loading. It is especially dangerous to load a cool cargo under warm and moist conditions. A moisture containing warm cargo loaded into a cold container, e.g. a reefer, is also a problem.

As the doors on the container are closed a certain amount of air is enclosed. Under normal conditions the amount of moisture contained in the air is usually insignificant in comparison with that which will be exchanged with the outside and the cargo during the voyage. But when loading under tropical conditions the amounts of moisture involved may be greater by a factor 10 or more, and special consideration will then need to be taken to quickly absorb the surplus moisture.

Is the Cargo well packaged and stuffed?

Even a completely sealed package may suffer moisture problems as a result of temperature variations.

Most packaging materials let moisture though and moisture will move both into and out of the packages during the voyage. This can be advantageous in a dry container, where the cargo will dry out into the container air. But it is of course a danger if there are moisture problems in the container.

A properly designed moisture protection considers the entire logistic chain and may involve a combination of absorbers placed within the cargo as well in the container and several layers of barriers with different properties chosen so that the net effect of the moisture migration is positive. To stop the moisture migration through using sealed alufoil bags is usually too expensive.

It is usually an advantage if the cargo is closely stuffed and there is as little free air as possible in the container. The pallet wrapping should have openings at least on the bottom. If mold or condensation is observed on the inside of the wrapping it is too tight.

Use InterDry Power Desiccant correctly

There are different types of absorbers with different properties. Calcium Chloride based absorbers are by far best suited for use in the container. For several reasons, perhaps most importantly because they do not risk re-evaporation of already absorbed moisture, the InterDry absorbers are by far the best and safest of the Calcium Chloride absorbers.

Other types of absorbers, so called “desiccants”, are more suitable for use inside packages. Whether one should have absorbers both in the container and within the packages, and in which proportions, depend on many considerations. Most importantly one must consider the entire logistic chain, and the need of protection before and after the container transport.

Generally, however, the container transport is the worst part of the logistic chain. It usually makes economic sense to provide as much of the moisture protection as possible in the packages. Using standard formulas to calculate the amounts otherwise required within the packages often lead to very large amounts of desiccants.

How many InterDry Bags or Poles are required depends on the cargo, how long the voyage is and what risk of damage is acceptable. For a dry cargo 4 Picollos 1000 are often sufficient in a 20 ft container.

Systematic Follow Up

Every transport is in some sense unique. The best way to optimize the moisture protection is to gather systematic feedback on all shipments.

Some things to note are:

Is the container dry. Are there any signs, e.g. discoloured carboard, bad smell, mold or insects, that indicate that the container at some point has been wet?

Is there any moisture damage on the packaging? On the cargo? In that case is there a pattern of damage – only on top, only in the centre of the cargo etc?

How much water has collected in the InterDry Absorbers? Is the amount about the same in all the absorbers or does it differ according to placement?

Temperature/weather at arrival.

To get much more information on what has happened during the voyage, it is possible to put an electronic logger in the container that records temperature and humidity throughout the voyage.

More regularly one can use the so called Alfasensor moisture indicators. They are a pair of simple stickers that can be put anywhere in the container, e.g. on an absorber or somewhere in the cargo. They will indicate how long the humidity has been above a certain set level giving much information about the voyage